undiminished (adj.)

prefix of negation, Old English un-, from Proto-Germanic *un- (source also of Old Saxon, Old Frisian, Old High German, German un-, Gothic un-, Dutch on-), from PIE *n- (source of Sanskrit a-, an- "not," Greek a-, an-, Old Irish an-, Latin in-), combining form of PIE root *ne- "not." Often euphemistic (such as untruth for "lie").

The most prolific of English prefixes, freely and widely used in Old English, where it forms more than 1,000 compounds. It underwent a mass extinction in early Middle English, but emerged with renewed vigor 16c. to form compounds with native and imported words. It disputes with Latin-derived cognate in- (1) the right to form the negation of certain words (indigestable/undigestable, etc.), and though both might be deployed in cooperation to indicate shades of meaning (unfamous/infamous), typically they are not.

It also makes words from phrases (such as uncalled-for, c. 1600; undreamed-of, 1630s; uncome-at-able, 1690s; unputdownable, 1947, of a book; un-in-one-breath-utterable, Ben Jonson; etc., but the habit is not restricted to un-; such as put-up-able-with, 1812). As a prefix in telegramese to replace not and save the cost of a word, it is attested by 1936.

early 15c., diminishen, "to lessen, make or seem to make smaller," from merger of two obsolete verbs, diminue and minish. 

Diminue (late 14c.) is from Old French diminuer "make small," from Latin diminuere "break into small pieces," variant of deminuere "lessen, diminish," from de- "completely" (see de-) + minuere "make small" (from PIE root *mei- (2) "small"). Minish (mid-14c.) is from Old French menuisier, from Latin minuere.

 Meaning "to lower in power, importance, or estimation" is from mid-15c. Intransitive sense of "to grow less, become or appear smaller" is from 1510s. Related: Diminishes; diminishing.